Page:The New International Encyclopædia 1st ed. v. 07.djvu/45

* EMBRYOLOGY. 31 EMBRYOLOGY. testine, and from it have arisen diverticula which give rise to the liver and pancreas. The hindgut gives rise to the rest of the la rye intestine and the rectum. By enlargement and growth of the amnion around the embryo until it joins the pedicle has been formed the body cavity. From this later are separated the pleural, pericardial, and peritoneal cavities. The primitive mo-geni- tal S3 r stem has appeared as a ridge along the dorsal wall of the body-cavity at either side of the median line in the posterior half of the embryo. This ridge contains the Wolffian duet and tubules and the rudiment of the Miillerian duct. There is no sex differentiation. which will ultimately form .a portion of the upper jaw; the main portion of the arch will form the lower jaw. The limb-buds have in - creased in size. The arm-bud projects from the cervical region, and I he leg-bud from the lumbar region. The yolk-sac is very small and the amniotic sac is much enlarged. Internally the nerves have begun to grow from the medullary tube or central nervous system into the mesen- chyma. Plexuses are forming, but the. nerves are not attached to their end-organs. Then' are now 38 primitive segments formed from the rnesen- chyma lateral to the spinal axis. Muscle-fibres are beginning to differentiate from the cells of Fourth Week. During the fourth week growth these segments, and their ventral ends grow into is relatively more active than at any other time The embryo about doubles in length. During the first part of the week the embryo becomes very much flexed, so much so that the head and tail nearly touch. The brain-vesicles are better de- veloped, as also are the gill-clefts and arches. the eyes, ears, and nasal pit -. FlS. 4. EMBRYO ONE MONTH OLD. a, a, a, cranial nerves: ar, auditory vesicle; h, bronchus; pbysis; k, kidney; /, liver; p. p, p, p, branchial pockets w.d, Wolffian duct. (After Mall.) tubular heart with its single cavity is rapidly changing into a complicated four-chambered structure with imperfect partitions. By the end of the fourth week ( Fig. 4 ), the anterior enlarged portion of the head has become bent at righl angles to the main axis of the body. From tfr first gill-arch a maxillary process is developing the primitive abdominal wall formed by the grow- ing around of the amnion. Condensed mesenchyma marks portions of the vertebral column. In the arm-bud the mesenchyma is beginning to differ- entiate into skeletal and premuscle tissue. The Wol Irian ridge has increased in size, and the The S-shaped Miillerian duct is now formed and runs parallel with the Wolffian duet. A diverticulum from the lower end of the Wolffian duct in- dicates the beginning of the permanent kidney and its duct. On the Wolffian ridge are also seen the first traces of the sexual glands, but not until the fifth week can sex be determined, even by micro- scopical examination. Fifth and Later Weeks. By the middle of the fifth week the embryo is nine milli- meters in length. The am- niotic sac is now so much en- larged that it is everywhere in contact with the chorion. A true umbilical cord of some length has developed from the abdominal pedicle attaching the embryo to the chorion as did the pedicle. The head is as large as the rest of the body, due to the rapid growth of the brain. The three pri- mary divisions of the brain are more marked and bent upon each other, and the cere- bral hemispheres have begun to grow. The spinal cord is a thick-walled tube. The va- rious cranial and spinal nerves now extend some distance into the body, and the motor nerves reach the premuscle masses of the head and shoulder. The gill clefts and arches are undergoing marked changes. From the first arch the up- per and lower jaws are de- veloping. From the first cleft the external auditory canal and ear-drum are form- ing, and from the pharyngeal portion of the cleft the Eustachian tube and middle ear; while the small bones of the ear arise from the dor- sal portion of the first gill-arch. From the second arch is forming the hyoid apparatus, and th^ remaining arches and clefts are beginning to disappear. The arm and leg are enlarged and c. cloaca j h. hypo- t, thyroid gland ;